A major percentage of the American population is myopic or nearsighted. Glasses and contact lenses have been the two most common approaches to correction of nearsightedness. Another approach is the radial keratotomy, whereby a number of precise incisions are made directly into the cornea of the eye in radial fashion about the central uncut optical zone of the cornea to relax its radius of curvature thereby increasing the focal length to correct or improve vision.
It will be appreciated, however, that radial keratotomies are extremely delicate procedures requiring extreme precision and safety. Eyes in general, and corneas in particular, are extremely sensitive and delicate. The cornea is a transparent lens tissue covering the iris and pupil through which light is admitted into the eye. The sizes and thicknesses of corneas vary from patient to patient. In performing a radial keratotomy, the length, width, depth and spacing of the incisions must be precisely controlled in order to accomplish the desired result without otherwise damaging the eye or cornea. This is complicated by the fact that eyes are subject to involuntary movements and are difficult to stabilize.
The history of radial keratotomy dates back to about 1869 when Snellen first described surgical methods for the correction of astigmatisms and raised the possibility of altering corneal curvature to correct vision. However, it was not until 1894 when Bates attempted to do just that by using a method called unsutured wedge resection. This essentially involved removing wedge-shaped sections of the cornea along radial lines centered on the visual axis.
In 1939, Sato reported on a series of cases in which incisions were performed on both the anterior (inside) and posterior (outside) surfaces of the cornea. The purpose of these incisions was to correct the preexisting irregular astigmatisms. His work in this field culminated in a paper presented in 1953 in which, once again, radiating incisions in both the anterior and posterior surfaces of the cornea were performed. Sato's incisions were intended to reduce a preexisting spherical myopia, i.e., no astigmatism. This procedure was successful with some of Sato's patients, however, unfortunately about 60-70% developed corneal edema or "fogging" of the corneal tissue.
Following Sato's work, others attempted to duplicate and improve on his basic concept. These individuals included the likes of Belyaev, Kio Tin and Yenaliev, among others. These procedures were relatively crude. Surgical loupes similar to jeweler's loupes were used for magnification. The incisions were performed with hand-honed, handheld unguarded surgical knives. The size of the optical zone, and the length and depth of the incisions, were estimated by eye.
It wasn't until the early 1970's that there were any significant breakthroughs in both the conceptual approach and surgical techniques for radial keratotomies. Fyodorov and Durnev demonstrated that permanent changes in corneal curvature could be induced by using partially penetrating radial incisions on only the anterior surface of the cornea. Fyodorov utilized high-powered stereoscopic operating microscopes to improve illumination and the amount and quality of magnification. High-carbon steel razor fragments were used for extremely clean incisions, and a micrometer adjustable blade holder with an attached quard were utilized to control the depth of the incision. Fyodorov also developed a device which, when pressed against the corneal surface, would leave a temporary circular mark to delineate the boundary of the optical zone. Corneal pachometers have since been developed for accurately measuring the thickness of corneas and, with the aid of computers, this information can in turn be used to map corneal topologies. However, despite these advances, radial keratotomies still tend to be regarded as experimental and unpredictable.
In contrast to mechanical cutting of tissue by surgical knives, high-intensity light sources such as lasers have heretofore been adapted to some forms of eye surgery, but have not been suitable for use in radial keratotomies for various reasons. Earlier applications of laser eye surgery involved mostly internal cauterizing and photocoagulation, instead of cutting. Photocoagulation devices are intended for treatment of the retina, and must therefore use radiation which penetrates the cornea, lens, and both humors without significant absorption and thus damage to these organs. Radial keratotomies require precision positioning of the beam and maximum absorption over a precise depth without penetrating the cornea tissue. Hand-held laser scalpels are completely impractical for such surgery.
Heretofore there has not been available a device incorporating a light source generating a beam of proper characteristics which could be steered and applied with sufficient precision and safety for radial keratotomy procedures. There is thus a need for an improved ophthalmic surgery system by which corneal keratotomies and resectomies can be performed with the precision, safety and reliability necessary in such procedures.